As low pollution fuel for vehicles, alcohol-mixed gasoline is used recently. The most appropriate air-fuel ratio of such mixed gasoline is different from that of gasoline itself. It is therefore essential to measure the amount of alcohol in the mixed gasoline, that is, the alcohol concentration in the mixed gasoline.
For measuring the alcohol concentration with high precision, it is preferred to use a parameter (physical constant), which has a relatively high change rate. For this reason, it is proposed to measure changes in the specific inductive capacity. Since the specific inductive capacity is calculated based on the change in the electrostatic capacity, JP 6-3313A, for example, proposes a liquid concentration measuring device, which has a pair of detection electrodes (detection electrode pair) provided to face each other to measure the static electric capacity. This liquid concentration measuring device further has a change-over switch and an electronic control circuit. The detection electrode pair is charged and discharged repeatedly at a fixed frequency by the control circuit through the change-over switch, so that a detection output voltage is produced in proportion to the concentration of liquid to be measured.
If the capacitance is measured by using the detection electrode pair, the resistance (leak resistance) between the electrodes becomes relatively smaller as the gasoline contains more impurities, that is, as the gasoline is of lower quality. Specifically, if the gasoline contains no impurities, it is in the insulated condition resulting in that the leak resistance becomes infinite and the conductivity between the electrodes becomes substantially zero. If the impurities increase, the conductivity becomes relatively high.
It is thus necessary to remove influence of the leak resistance to measure the concentration of liquid such as alcohol precisely.
It is therefore an object of the present invention to provide a liquid concentration measuring device, which can measure a liquid concentration without being influenced by a leak resistance developed in a detection electrode pair.
According to one aspect of the present invention, a liquid concentration measuring device comprises a detection electrode pair, a switch section, an operation signal outputting section and a measured value outputting section. The detection electrode pair includes a pair of electrodes facing each other and disposed in a liquid to be measured. The switching section is provided to switch over charging and discharging of the detection electrode pair. The operation signal outputting section is configured to produce operation signals of a predetermined frequency to control switching operation of the switching section at a predetermined period. The measured value outputting section is configured to produce a voltage for charging the detection electrode pair through the switching section and to produce a detection voltage corresponding to a static electric capacity of the detection electrode pair. The operation signal outputting section is configured to produce a first operation signal of a first frequency and a second operation signal of a second frequency for operating the switching section at a first period and a second period, respectively. The measured value outputting section is configured to produce a first detection voltage and a second detection voltage when the switching section is operated at the first frequency and the second frequency, respectively.